1. Field of the Invention
This invention relates to a process and system for regeneration of an adsorbent used for removal of contaminants from fluid streams, in particular, for removal of volatile organic compounds (VOC's) and/or hazardous air pollutants (HAP's) by adsorption using a suitable adsorbent.
2. Description of Prior Art
Adsorption/regeneration processes in which an adsorber is cyclically operated first to adsorb a given material contained in a fluid stream passed through the adsorbent and then to give up the adsorbed material to a regeneration fluid passed through the adsorbent resulting in regeneration of the adsorbent and rendering the adsorbent suitable for additional adsorption of material are known. In a known system for solvent recovery, activated carbon is employed as an adsorbent through which an air stream containing the solvent is passed. The solvent is adsorbed by the activated carbon after which the saturated adsorbent is contacted with steam, which steam provides the heat of desorption, resulting in the solvent being desorbed from the carbon. The condensate containing the solvent is then distilled to recover the solvents and the condensate is then discharged. However, contaminants comprising water soluble solvents such as water-based ink preclude the use of a steam based system because the solvent is no longer distillable from the condensate and, thus, the condensate can no longer be discharged without further costly treatment for environmental reasons. The use of air as a substitute for steam for regeneration of carbon adsorbents is possible but is generally hazardous, particularly with respect to solvents containing methylethyl ketone, acetone and other very reactive hydrocarbons which would ignite on the carbon due to the exothermic reaction resulting from reactivity of activated carbon in the presence of oxygen at moderate temperatures. In order to use activated carbon under such conditions, use of inert gas is necessary, making such systems uneconomical.
Additional disadvantages of known adsorption/regeneration processes and systems include the requirement for substantial amounts of energy to heat the desorption fluid and dispose of the desorbed material in an environmentally acceptable manner.
To overcome certain of the problems associated with the use of activated carbon as an adsorbent, Japanese Patent 0,015,657 teaches a method for adsorption of volatile organic compounds using zeolite followed by desorption and incineration of the desorbed organic fumes. Similarly, U.S. Pat. No. 4,846,852 teaches a method for separating and recovering volatile solvents from exhaust air in which the exhaust air containing the volatile solvents is alternately flowed through two molecular sieve packed beds, adsorbing the solvents therein, after which heated air or inert gas is passed through the bed not used for adsorption to regenerate the molecular sieve. Finally U.S. Pat. No. 4,820,318 teaches a process for removal of volatile organic compounds from vapor streams using an adsorbent of hydrophobic molecular sieve which is subsequently regenerated by heating. As a result of the use of molecular sieves as adsorbents, which molecular sieves are made of silica and alumina oxides, the danger of fires is eliminated, thereby enabling use of air, as opposed to steam, for regeneration of the adsorbent.
U.S. Pat. No. 4,784,672 teaches a process for removal of trace hydrocarbon impurities, water and hydrogen sulfide from landfill gas using an integrated temperature swing adsorption-pressure process with an adsorbent of a layer of activated carbon over a layer of molecular sieve zeolite. U.S. Pat. No. 4,812,147 teaches a process for removal of multiple components from a gaseous stream by adsorption in which the gaseous stream is passed sequentially through separate and distinct adsorbent beds wherein only a portion of the more strongly sorbed component is removed in the first adsorbent bed and the remaining portion of the more strongly sorbed component and a second component are substantially adsorbed in the second adsorbent bed. Regeneration of the adsorbent beds is accomplished using an inert purge gas.
U.S. Pat. No. 5,122,165 teaches a process for removal of VOC's from liquid streams in a process system utilizing, among other components, a self-generative gas phase granular activated carbon contacted for collection of the VOC's from a gas effluent which has previously been purified by a foam collector, wet scrubber, demister pad and VOC condenser. Processes for producing carbon molecular sieves are taught by U.S. Pat. No. 4,902,312 and U.S. Pat. No. 5,164,355. U.S. Pat. No. 5,071,450 teaches a process for modifying/treating molecular sieve adsorbent in which carbon is deposited on the adsorbent in order to change the effective diameter of its micropore openings.
In known adsorption/regeneration processes in which molecular sieves and/or high silica zeolites are used to adsorb VOC's, followed by desorption/regeneration of the molecular sieve using air, the VOC's in the regeneration stream are typically incinerated. See Japanese Patent 0,015,657. However, such processes require substantial amounts of fuel for incineration of the VOC's.